1. Field of the Invention
The present invention relates to a sensor driven contactlessly by a single power supply, or more in particular to a sensor driven contactlessly by a single power supply adapted for enlarging the range in which the voltage or current is measured in contactless fashion by driving a single power supply.
2. Prior Art
Explanation will be made below with reference to a contactless voltage sensor or a contactless current sensor used with an electric automotive vehicle.
An electric automotive vehicle is driven by a battery of cells, and the driver of the electric automotive vehicle decides on the remaining distance coverable by observing the residual capacity of the battery on display. The estimation of the residual capacity, therefore, requires a very high precision.
For this reason, the contactless measurement of a voltage or a current generally uses a servo type suitable for high-precision measurement of all the measurement methods including the resistance type, CT type and the servo type. A contactless current sensor of servo type with a single power supply will be described below.
A contactless sensor driven by a single power supply is disclosed in JU-A-4-24070. This current sensor performs the sensing operation with a single power supply.
The provision of a single power supply, however, limits the measurement range to the capacity of the particular single power supply. In other words, the size of the measurement range of the contactless current sensor driven by a single power supply described in JU-A-4-24070 is equivalent to the voltage Vcc of the single power supply.
The specific measurement range can be set between 0 and +Vcc, between -Vcc/2 and +Vcc/2 or between -Vcc and 0 as desired to meet the requirement of each case of power generation or charging operation by effective use of a variable resistor. Even when a variable resistor is used, however, it is not an easy matter to change the reference voltage manually in accordance with the timing of the positive or negative direction of current.
Especially in the case where it is required to detect the charging such as when a regeneration current is generated, the problem is posed that the measurement range cannot be changed unless the resistance value is switched each time.